1. Field of the Invention
The present invention relates to a method of corrosion control for buried piping, and in particular to an impressed current cathodic protection system for protection of isolated and electrically grounded buried or submerged metallic structures, an electrical means of anti-cross-connection, a means of cross-connection warning, a means of controlling DC voltage and current output on a cathodic protection system, a means of maintaining a low resistance ground-bed, and a means of anode and backfill installation and transport.
2. Discussion of Related Art
Cathodic protection is a widely used method of corrosion control for buried or submerged structures. Systems for the cathodic protection of buried or submerged structures are generally well known. Examples include U.S. Pat. Nos. 6,471,851, 6,461,082, 634,188, and 6,315,876.
There are two basic methods of cathodic protection, including the passive, or sacrificial anode system, and the active, or impressed-current system. Each type of system achieves the same purpose, to provide cathodic protection to a structure that is being protected.
Whether using an active or passive system, to achieve full protection often requires using dielectric insulators to electrically isolate the protected structure from other structures that are not intended to be protected. Electrical isolation of the protected structure allows the protective current to be contained within the desired circuit, and not “lost” to other buried or submerged structures that are not intended to be protected. Electrical isolation of the protected structure assists in achieving reasonable system life expectancy by keeping the anode-bed current at lower and more manageable levels. There are cases where full cathodic protection cannot be achieved without electrically isolating the structure to be protected.
With regard to the application of cathodic protection for buried piping associated with commercial or residential buildings, electrical isolation of the piping is practically impossible, and not legal in many cases. With typical building construction, the piping is buried below the concrete foundation and flooring slab, and projects upwardly through the concrete slab at numerous service locations within the building inside walls and concealed spaces, creating access problems and numerous potential areas for electrical contact, or “shorting”, to the building grounding system. Therefore, an attempt to isolate below-slab metallic piping is usually not practical due to the many inaccessible locations that would require dielectric insulators.
Legally, in many if not all cases, it would be against building codes and regulations to electrically isolate piping within a commercial or residential building. This is due to safety issues related to the electrical grounding requirements, where the piping is grounded to provide a low resistance path to earth for the electric circuit-breakers to work. In other words, if the piping were not grounded it could be electrified with high voltage and the circuit-breaker would not pop open, until some conducting member or structure completed the circuit to ground. The conducting structure could be a living creature such as a human being and completion of the electrical circuit would generally lead to electrocution.
Therefore, there is a need to provide a cathodic protection system that is well suited to provide protection to buried piping systems that cannot be electrically isolated and that are electrically associated with the electrical grounding system of the utility company.
With the active, or sacrificial anode system, the electrical current is driven by the naturally occurring fixed voltage between the anode material (typically zinc or magnesium) and the structure to be protected (usually steel or copper). This fixed voltage limits the current output, especially in higher resistance soils, and often makes the active system an expensive and generally poor choice for protection of bare or poorly coated structures that are not able to be electrically isolated.
The active, impressed current system, described herein may provide a useful alternative to the application of cathodic protection for buried or submerged structures that are shorted to the electrical grounding systems, particularly piping systems typically found on commercial or residential buildings. (Such an impressed current DC power supply unit is herein referred to as a cathodic protection rectifier, or, more simply a “rectifier”. The power supply unit may also be connected to an alternating current AC supply and thus a transformer may be provided to convert the AC into DC).
The cathodic protection system described here may provide a system with inherent safety features to guard against cross-connecting the structure and the anode-bed. In the cathodic protection installation and service industry, a cross-connection of the output leads is a very serious and real concern. In a cross-connection incident, an operator accidentally connects the cathodic protection rectifier output wires in the wrong positions, connecting the anode lead to the negative (−) terminal and the structure lead to the positive (+) terminal. This results in the structure being oxidized at an accelerated rate, and the consumable anode(s) being protected; the exact opposite of what is trying to be accomplished with a typically expensive cathodic protection system. This is especially bad if the structure that is damaged transports or stores flammable gas or petroleum oil.
A method of DC voltage control and regulation is described which is highly suitable for protection of piping in residential and commercial buildings. A method of maintaining a lower resistance anode bed is described. Also, a method of anode and backfill installation and transport is described that makes cathodic protection a more practical alternative to ongoing corrosion problems.
The present invention is designed to be easier to install, easier to service, more reliable, safer, more versatile in application (able to protect various grounded piping systems), and lends itself to do-it-yourself installers. The safety and transport aspects are particularly important for a reduction in cost to the consumer.